1. Field of the Invention
The present invention relates to a stage apparatus used in precision equipment, such as semiconductor exposure systems, for moving and positioning a substrate, such as a semiconductor wafer and an original for exposure.
2. Description of the Related Art
In semiconductor manufacturing processes, projection exposure systems are used for projecting and transferring a pattern formed on a reticle substrate, which is an original, onto a silicon wafer, which is an exposure target. When the reticle pattern is projected onto the silicon wafer, the reticle and the silicon wafer are moved with respect to a projection exposure unit using stage apparatuses, such as a reticle stage and a wafer stage.
FIGS. 5 to 9 show the construction and the operation of a known projection exposure system. FIG. 5 is a schematic diagram showing the overall construction of the projection exposure system. With reference to FIG. 5, an illumination unit 1 irradiates a reticle (not shown) with exposure light emitted from an exposure light source (not shown) after the shape thereof is adjusted. The reticle, which is an original with an exposure pattern formed thereon, is placed on a reticle stage 2 and is moved with respect to a wafer 8 (see FIG. 6), which is an exposure target, at a predetermined reduction exposure ratio in a scanning process. A reduction projection lens 3 reduces and projects the original pattern formed on the reticle onto the wafer 8. A main unit 4 of the exposure system supports the reticle stage 2, the reduction projection lens 3, and a wafer stage 5. The wafer stage 5 moves the wafer 8 stepwise to different exposure locations, and also moves the wafer 8 in synchronization with the reticle in the scanning process.
With reference to FIG. 6, the wafer stage 5 includes a stage base 5D and a slider (moving unit) 5C. The wafer stage 5 has an illumination sensor 5A and a stage reference mark 5B on the top surface thereof (on the top surface of the slider 5C). The illumination sensor 5A is used for a calibration measurement of the illumination of the exposure light, which is performed before exposure in order to correct the amount of exposure light. The stage reference mark 5B includes a target for stage alignment measurement.
As shown in FIG. 7B, which is a sectional view of the wafer stage 5, the slider 5C has a surface-motor driver coil 5F, which is disposed in the slider 5C, and which drives the slider 5C along the top surface (reference surface) of the stage base 5D. An iron-core comb yoke 5E is provided on the stage base 5D such that the iron-core comb yoke 5E faces the surface-motor driver coil 5F, and the slider 5C is moved along a two-dimensional XY plane above the stage base 5D due to the interaction between a yoke, which is magnetized by the surface-motor driver coil 5F, and the iron-core comb yoke 5E.
FIG. 8 is a sectional view showing the detailed construction of the slider 5C. With reference to FIG. 8, an air bearing 5G, which is a static bearing, supports the slider 5C, such that the slider 5C can move along the XY plane, and supply air 5H is supplied to the air bearing 5G so that a static force can be generated. A six-axis fine motion stage 5N is mounted on top of the slider 5C to finely move the wafer 8 in X, Y, Z, θx, θy, and θz directions, and positioning and focus/tilt adjustment of the wafer 8 are performed during exposure using the six-axis fine motion stage 5N. Heat 5J is emitted from the surface-motor driver coil 5F, and exhaust air 5K is discharged from the air bearing 5G. In addition, a temperature increase 5Q occurs due to the heat 5J emitted from the surface-motor driver coil 5F. As used here, the term “air” is intended to cover not only cleaned and dried atmospheric air, but also, inert gases, such as nitrogen gas and helium gas, and a mixture of inert gases and atmospheric air.
With reference to FIGS. 5 and 6 again, the focus scopes 6 are integrated with a lens barrel of the reduction projection lens 3 and are used for focus measurement of the wafer 8. An alignment scope 6A detects an alignment mark (not shown) formed on the wafer 8 and the stage reference mark 5B formed on the wafer stage, and performs measurements required for the alignment of each shot location in the wafer 8 and the alignment between the reticle and the wafer 8. A wafer conveyor robot 7 conveys the wafer 8 to the wafer stage 5. The wafer 8 is constructed by applying a resist onto a single-crystal silicon substrate, and the reticle pattern formed on the reticle is projected and transferred onto the wafer 8 through a reduction exposure unit, such as reduction projection lens 3 shown in FIG. 5.
A plurality of laser interferometers (not shown) are mounted on the slider 5C (see FIG. 6) for measuring the position of the slider 5C (hereafter, called the position of the wafer stage 5), that is, the position of the wafer 8. An X interferometer mirror 9 (see FIG. 6) serves as a target used by an X laser interferometer for measuring the position of the wafer stage 5 in the X direction, and a measurement beam 9A (see FIG. 7A) is emitted from the X laser interferometer. A Y interferometer mirror 10 serves as a target used by a Y laser interferometer for measuring the position of the wafer stage 5 in the Y direction, and a measurement beam 10A is emitted from the Y laser interferometer. Although the mirrors 9 and 10 are disposed outside the moving unit in FIG. 6, the construction may also be such that the mirrors are mounted on the moving unit so that lasers are externally directed onto them.
The inventors of the present invention have found that the stage positioning accuracy of the above-described known structure is below a level which can be expected, in view of the constructions of the position measurement unit and the stage driver unit, and that the accuracy can be improved. The inventors of the present invention have analyzed the cause of the degradation of control accuracy in the known structure and have found the facts described below.
That is, when the movement of the slider 5C is controlled, as shown in FIG. 9, in the exposure system shown in FIGS. 5 to 8, a drive current is applied to the surface-motor driver coil 5F (see FIG. 8), and the temperature of the slider 5C increases accordingly. When the temperature of the slider 5C increases, the temperature of the air 5H, which flows through the air supply unit of the air bearing 5G, also increases. Therefore, the temperature of the exhaust air 5K and that of the air surrounding the slider 5C increase. As a result, air fluctuation occurs, which adversely affects the measurement beam 9A from the X laser interferometer and the measurement beam 10A from the Y laser interferometer (each shown in FIG. 7A), causing measurement errors.
In addition, when the temperature of the slider 5C increases, heat is transmitted to the six-axis fine motion stage 5N mounted on top of the slider 5C, and the temperature increase 5Q occurs in the six-axis fine motion stage 5N. Accordingly, thermal distortion of the six-axis fine motion stage 5N and a wafer support, which supports the wafer 8, occurs.
As a result, when the slider 5C is moved to a desired position on the basis of measurement values obtained by the laser interferometers, the slider 5C cannot be accurately positioned at the desired position, and the control accuracy of the stage apparatus is degraded. In addition, the wafer flatness is reduced and the focusing accuracy is degraded accordingly. Thus, the overall performance of the exposure system is degraded.